Polymeric materials in sheet or film form containing fillers and are suitable for use in the electronics industry in the manufacture of electronic substrates, chip packages, circuit boards and other electronic devices or in the separating technologies where filters, separators or membranes are required. The ultimate use of the film and the particular property that the film will exhibit determines the selection of the specific filler. For example, activated carbon can be incorporated into the film to provide a filter or separator. Electrical properties can be attributed to a polymer film by incorporating fine metal particulates to impart electrical conductivity. Frequently, the film is a polymeric adhesive and the metal particles provide the film with the requisite electrical property. Thermal conductivity can be obtained by adding a ceramic and/or metal and/or diamond into the adhesive.
Adhesive films can be formed from (1) a solution or paste that will polymerize to form the "polymeric" film, or (2) a polymeric substrate having an adhesive added thereto. In either case, as the quantity of filler is increased to provide the desired electrical/thermal property, the physical properties of the adhesive are degraded to a point of limited utility. For example, a typical conductive adhesive might have as much as 40-60% (v/v) filler. However, these adhesives are very weak and brittle, and are only useful when dispensed as a paste/liquid. If a film can be made from these compositions, they are difficult to handle in the "B" staged form, limiting their size and thickness.
One approach to overcome the shortcomings of filled adhesives is to limit the quantity of the filler that is added. A separate approach is to add a reinforcement material like woven glass fiber. However, in both of these approaches, the performance of the filled adhesive is severely compromised when this is done. As a result, less than the desired amount of filler material is used. The resulting sheet adhesive can be worked, but the desired or optimal property and performance are not obtainable. Thus, existing filled adhesives sacrifice performance for usability.
In addition, particle filled sheet adhesives suffer from a phenomenon known as "particle settling" or sedimentation. The heavy particles (up to 10.times. the density of the resin) will sink to the bottom of the film, leaving a resin rich surface. This phenomenon results in undesirable inhomogeneities and poor reliability. High bond pressures are often required to insure that the excess surface resin is pushed back into the film. Even so, surface irregularities, could still result in resin rich areas that would be deficient in the property supplied by the particle.
Attempts at preparing filled film adhesives have been made, but significant drawbacks exist and therefore their preparation are limited. The quantity of filler that can be added is controlled by the physical limitations of: (1) polymeric film or substrate; and (2) the filler-adhesive additive. Often, the desired quantity of filler renders the adhesive-filler additive impossible to handle.
Thermosetting resins have been imbibed into substrates to impart structural integrity to an otherwise brittle layer. For example, a woven glass substrate can be imbibed with a thermosetting or thermoplastic adhesive. However, this approach has significant drawbacks when a filler is also added. For example, hollow glass microspheres have been dispersed into a thermosetting adhesive and then imbibed into a woven substrate. However, the amount of filler that can be delivered to the substrate is limited because the flexibility and manipulability of the resulting adhesive film is poor and it is difficult to conform the adhesive film for the desired applications.
Another drawback to such a composite is the limited homogeneity possible with the woven structure. At every interstice of the weave lies a composition that is different than the volume defined between the interstices. This results in inhomogeneities in physical properties, electrical properties, and in conformability. It would be desirable to have a filled adhesive sheet that is homogeneous and flexible at wide ranges of filler level.
Also, although curing additives have been added into a pressure-sensitive adhesive, and then imbibed into a polyurethane foam, the nature of the scaffolding, poses many limitations. As a result, it is very difficult to make thin composites, or flexible, thermally stable composites. Flame retardant particles have also been dispersed into an adhesive and then imbibed into a non-woven polyimide-ester substrate for use as a flame-retarded flexible circuit substrate. In general, in these prior art systems, dispersing an additive into an adhesive at an optimal level for performance sake and at the same time providing that adhesive as a thin sheet is not feasible.
Fillers have also been incorporated into fluoropolymers, such as porous, expanded polytetrafluoro-ethylene (ePTFE), by directly adding the filler to the fluoropolymer prior to extrusion and expansion. Thin porous polytetrafluoroethylene films filled with inorganic filler that are between 0.1 and 5.0 mils thick and substantially pin hole free are also known, as are thin capacitive polytetrafluoroethylene layers for printed wire circuit boards (PWB). The porous expanded polytetra-fluoroethylene used can be prepared in accordance with the teachings of U.S. Pat. No. 3,953,566 to Gore. In each case, the filler is admixed with the polymer prior to extrusion and expansion.
Polytetrafluoroethylene that has not been expanded, and thus does not include the fibril-node micro-structure of expanded polytetrafluoroethylene, can also include fillers. Extruded composite tapes of ceramic filler and polytetrafluoroethylene with dielectric properties have been prepared by adding the filler to the polytetrafluoroethylene prior to extrusion. Silane compounds can also be desirable for providing uniformity and improved dispersibility when aqueous polytetrafluoroethylene-filler dispersions are prepared. See U.S. Pat. No. 3,929,721 to Leverett and U.S. Pat. No. 4,038,244 to Ogden, et al. In addition to silanes, other organic compounds that render hydrophilic fillers hydrophobic, for better dispersibility, are also known. See U.S. Pat. No. 4,440,879 to Kawachi, et al. and U.S. Pat. No. 4,143,110 to Morozumi. It is also known to precoat filler with a coupling agent, e.g. a silane, titanate, etc. prior to paste extrusion
Composites formed in these patents require bonding at "sintering" temperatures--i.e., at very high temperatures or at high temperature and pressure. As a result, these composites have not been used to the extent contemplated, especially because of the difficulty encountered when the composites are further processed. This is particularly the case when the other materials being bonded to these composites cannot withstand these elevated temperatures and pressures.
Other attempts to avoid these drawbacks have been made, but the results have been unsatisfactory. Porous fibrillated polytetrafluoroethylene has been prepared according to the technique taught in U.S. Pat. Nos. 3,407,096 and 3,407,249 to Landi. In the Landi patents, inorganic or organic fillers are incorporated into the network of unsintered polytetrafluoroethylene fibers. The Landi process involves preparing a blend of polytetrafluoroethylene and an organic polymer that is extruded. The organic polymer is subsequently removed by dissolving in an appropriate solvent. The resulting structure precludes the introduction of additional particulate fillers because of the very fine fibril network.
In U.S. Pat. No. 5,141,972 to Sato, gas-containing microballoons or spheres are used to form an insulating porous composite with polytetrafluoroethylene. In the Sato patent, the polytetrafluoroethylene substrate has a porosity of about 75% and is dipped into an ultrasonically-stirred liquid containing microballoons, allowing the spheres to flow into the pores. The imbibed substrate is heated without restraint, thus shrinking the porous polytetrafluoroethylene so as to fix the microspheres in the pores. The product produced, according to the Sato patent, is useful as a porous compressive-resistant, low dielectric. The spheres fill the pores and thus prevent the pores from being collapsed. Using the same approach, in U.S. Pat. No. 5,087,641 to Sato, porous polytetrafluoroethylene composites with sintered polytetrafluoroethylene particles within the pore volume are fabricated. In each case, the Sato composites cannot satisfactorily be imbibed with resin, and as such, would be very difficult to use as an adhesive.
Although the prior art has produced substrates that contain fillers, the addition of an adhesive to the particle being produced has not resulted in acceptable products. For example, in Japanese Laid-open-patent application 61-40328 to S. Hamasaki, et al., silicone rubber is imbibed into a porous expanded polytetrafluoro-ethylene structure for use as a thin electrical insulator with a thickness not greater than 50 mm. The silicone rubber is imbibed as a solution and results in a product that is transparent, e.g., free of filler, that is subsequently cured. However, the structural integrity of the cured product is poor. In an attempt to reinforce the structure of the Hamasaki patent, H. Kato, et al., in Japanese Laid-open-patent 62-100539, teach a silicone rubber article which is made by first incorporating a ceramic into a dispersion of polytetrafluoroethylene. The filler is incorporated directly into the nodes of the node-and-fibril structure, and thereafter the silicone resin is imbibed into said fibrillated structure as described above. In both of these instances, the final product is a rubber-like cured sheet.
In a similar fashion, M. Hatakayama, et al., in GB-2195269B (EP-0248617B1), describes an article and process of imbibing expanded polytetrafluoroethylene with a thermosetting resin which is useful as an adhesive for printed wiring boards (PWB). An inorganic filler could be incorporated into the node-fibril structure of the expanded porous polytetrafluoroethylene. See also U.S. Pat. No. 4,784,901 to Hatakayama, et al. who impregnates a resin without filler into polytetrafluoroethylene.
Unfortunately, with these approaches, it is difficult to attain high degrees of ceramic loading because the ceramic serves to weaken the node and fibril structure. It is difficult to make thin films of filler-reinforced polytetrafluoroethylene because as the material thickness is reduced, the filler/fiber particulate creates pinhole tears. Furthermore, incorporating inorganic fillers, especially in the range greater than 30 volume percent, makes mixing and paste extrusion processing of these composites very difficult. An additional misfortune is that the ceramic reinforcement is not uniformly distributed throughout the composite structure.
A need exists for a uniformly reinforced, thin adhesive sheet composite that is capable of retaining high levels of reinforcement. Thus, a need exists for structurally reliable, filled-adhesive films that contain the greatest possible quantity of filler to maximize the desired property without sacrificing structural integrity. These films should: be as thin or as thick as possible, be in an easy-to-use, convenient sheet form which is not brittle, have uniform consistency and be pinhole free.
The subject invention, described below, overcomes these and other drawbacks of the prior art.